I initially intended to write a review of this book, but that part is easy. It’s great! Aside from one minor complaint, I loved it and I think everyone should read it.
Instead, I’d like to share some of my takeaways to peak your interest and hopefully start a discussion.
#1 – LF is hard to control
Loud bass is important to rock concerts. It’s also very difficult to control and requires the largest share of electric amplification energy.
Bass is so hard to control because the wavelengths are so long. They go everywhere. This causes various problems.
The LF directivity of loudspeakers is so low that the critical distance becomes very short, puts the majority of the audience in reverberation, and partially masks higher frequency direct sound.
Unfortunately, the presence of an audience doesn’t help much. There’s a common belief that when the room fills with people that the reverb will dry up and even out, but that may only be true at mid and high frequencies.
The presence of an audience does not have a big influence on the low frequency reverberation time of a hall. This is one reason why halls for pop and rock must be designed with a low RT at low frequencies.Adelman-Larsen, Niels. Rock and Pop Venues. Cham, Switzerland, Springer Nature, 2021.
What’s the solution? Appropriate architecture to control the reverb time (RT) and an optimized sound system to keep sound off of the walls.
It is a fact that the more sound the PA system shoots onto the walls and ceiling, the more the reverberation of the hall is evoked.
#2 – Ideal reverb time is based on volume
The author provides a graph with target reverb times based on room volume and makes this generalization.
The halls investigated in this book are in almost all cases over 1000 m³. With an ideal reverberation time for pop and rock music of 0.6 seconds for that size volume this yields a Schroeder frequency below about 50 Hz.
#3 – Rooms must be designed for low RT at LF
This one factor, low RT at LF, has to be fulfilled in making a good sounding hall for pop and rock music. This is the single most important message of this book.
The 125Hz octave band is the most important to control because:
- Directivity of loudspeakers increases with frequency, and so does critical distance.
- Absorption by audience increases with frequency.
- The 125 Hz band is louder and busier than the 63 Hz band.
- The human hearing system shows little upward masking in the 63 Hz band.
One octave centered at 125Hz = 88-177Hz. What’s happening in that area? The alignment between main and sub arrays, especially with common haystacking. If you get it wrong, you may end up breaking the first rule of sound system design and spraying the walls with sound instead of the people. For a practical guide to sub alignment, please see 6 Top Brands, 1 Sub Alignment Method.
The ideal conditions for concerts include optimized speaker directivity in bigger rooms with more absorption, lower RT, and less sound sources.
This means that a larger share of the audience will enjoy a clear sound when Q [directivity], V [volume], and a [area weighted average of the absorption coefficient] ar increased and when T [reverberation time] and N [number of loudspeakers or clusters] are decreased.
And one final recommendation from the author about placing absorption.
In the design phase, not a floor, not a wall, nor the ceiling must be left without considering low frequency absorption.
Let’s get my one minor complaint out of the way.
A challenge with the cardioid configuration can be to match the compensating delay exactly with the physical separation of the subwoofers. The location of the acoustic center of the subwoofer can vary with frequency, making it difficult, if not impossible, to find one delay setting that suits all frequencies. Failure to match delay and physical separation fully can result in significant radiation backward within certain frequency bands, which can represent a problem, for instance for the musicians on stage. All-pass filters can be used here to make phase adjustments at specific frequencies.Adelman-Larsen, Niels. Rock and Pop Venues. Cham, Switzerland, Springer Nature, 2021.
This is an unnecessary complication of an otherwise simple design and calibration task. I’ve never had a problem calibrating directional subwoofer arrays in the field with delay only. Of course, further optimization could be made, but we’re talking about 1-10º. I asked several colleagues that I trust about this and no one had ever heard of the idea.
Here’s an example alignment of an inline gradient array using delay only. The polarity inversion has been removed to facilitate comparison.
Here’s the same array, this time aligned with an APF.
Here’s the APF I used.
Have you tried building a directional subwoofer array using APF? What were your results?